IBM Selectric Maintenance Manual page 53

Since this .260" to .270" is measured between the
typehead and the platen it is not a measurement of
the powered travel of the typehead. The .020" to
.030" free flight is included in this .260" to .270"
and must be subtracted in order to determine the
amount of powered travel. The powered travel must
never exceed .265" as this will cause typehead
breakage.
The base of the Hooverometer handle can be used
as a measuring devi ce as illustrated in Fig. 95. The
handle base is about
.250"
thick; however they do
not all measure the same. Each handle should be
measured with"a dial indicator or micrometer to de-
termine its exact size. When the size of the handle
base is determined, a thickness of tab cards should
be placed around the platen that will total .260"
to .270" when added to the thickness of the handle
base.
e. Both adjustments must be rechecked and refined un-
ti I these conditions are obtained. Be sure to tighten
both screws firmly when the adjustments are com-
pleted.
NOTE: The adjustments should be checked with
the ri bbon removed.
Print Cam
Piint Com follower
FIGURE 95. Velocity Control Plate Adjustment
The print cam has a fixed amount of rise from its low
point to its high point. For this reason, the print cam
follower always receives the same amount of powered
travel or motion from the print cam. However, the
a-
mount of powered travel that the rocker and typehead
receive from the print cam follower is directly dependent
upon the position of the velocity control plate pin in the
forked slot of the follower (Fig. 95). Moving the pin to
the front of the slot decreases the powered travel of the
type head as shown by dimension A (Fig. 95). Moving it
to the rear increases the powered trave I, as shown by
di mensi on B.
Because of the constant operational speed of the print
cam and follower, the typehead velocity must increase
or decrease proportional to the increase or decrease in
powered travel. An accompanying change in typehead
velocity occurs when the powered travel is changed be-
cause the type head must always trave I from its rest posi-
tion to its active position in the same amount of time re-
2-30
gardless of what this distance may be.
The proper impact velocity of the typehead can be
achieved by controlling the amount of powered travel of
the typehead while maintaining at the same time the
correct amount of free flight. Since the position of the
platen has been previously fixed and the amount of free
flight is determined by the point at which the limit of
powered travel occurs relative to the platen, on Iy the
rest position (beginning of powered travel) may change
wheA the amount of powered trave I is changed. There-
fore, in order to maintain a fixed amount of free flight
to the type head when the amount of powered travel is
changed, the velocity control plate pin must also be ad-
justed up or down relative to the rocker.
The eccentric shouldered nut (Fig. 93) on the velocity
control plate provides a means of controlling the vertical
position of the velocity control plate pin relative to the
rocker thereby controlling the free flight of the type-
head. The elongated hole in the velocity control plate
(behind the binding screw) allows the velocity control
plate pin to be adjusted front or rear in the forked slot
of the follower thereby controlling the amount of power-
ed travel that the typehead wi II receive. Each adj ust-
ment affects the· other and must be adjusted alternately
unti I both are correct.
These adjustments are designed to produce optimum
pri nt qua Ii ty for most app Ii cati ons; however, a cer-
tain amount of variation in velocity is permissible
in order to satisfy"a customer's application. Be
cautious of creating an excessive increase or re-
duction in velocity as this will adversely affect the
uniformity of impression between characters and the
genera I appearance of the printed copy.
7. Anvil - The eccentric at each end of the anvil is ad-
justed so that the anvil properly restricts the free flight
of the typehead. The adjustment can be checked by
printing a period or similar character with the copy con-
trel lever set all the way back. The period shouid raii
to print or print very faintly. With the copy control lev-
er pulled forward one notch (fourth position), the period
should print lightly. The ribbon and one sheet of paper
should be used when making the check. The check should
be made at each end of the platen only; because flexing
of the parts will allow a slightly different condition inthe
middle of the platen. The high part of each eccentric
should be kept outward towards the side (Fig .96).
NOTE: The restoring cam follower eccentric should be
adjusted all the way up while the anvil is being set. The
carrier buffers must also be moved up out of the way of
the anvil (Fig. 96).
The free flight must be restricted to insure that the char-
acters wi II a II emboss the paper to the same depth. If
no restriction were applied, the smaller characters such
as punctuation marks would be allowed to emboss too
deeply. The surface area somewhat restri cts the amount
of embossing for the larger characters; thus if the free
flight of the rocker is stopped at the right point, even
impression between all characters will result.
CAUTION: Raising or lowering either end of the anvil